The exposed plastic on the outside should be fairly flush with the boat by the time you're done so no, it's not an issue. In fact, it usually smooths and buffs out as you run over some rocks. When I asked this on the buzz, mostly people seemed to say weld it on the outside, then weld it on the inside, then slap some bituthene on it for the ultimate patch. I usually do outside, then inside, then put some gorilla tape over it (easier to get off than bituthene if needed). I think welding the inside is probably not necessary, but it's nice to put some bituthene or tape on the inside in case it opens back up on you while you're no the water. If you're in a hurry or getting lots of little cracks, just weld the outside and you won't have to deal the pain of taking out outfitting.
Btw - I usually just wear a leather glove and take some wax paper and smooth out the melted plastic by hand.
Tom, could you intersperse some SSV boat cracking footage into that video so it's less mind numbingly boring. And how does your house not smell like burnt plastic? I try to do it outside, but I've done it in the garage and it smells like plastic for a day - can't be good for you.
I definitely do it more like the LL guy, but Tom's welds seem to hold amazingly well. I think maybe small crack, do it LL style, big crack TJ style?